The prior art teaches equipping vehicles with “variable displacement,” “displacement on demand,” or “multiple displacement” internal combustion engines in which one or more cylinders may be selectively “deactivated,” for example, to improve vehicle fuel economy when operating under relatively low-load conditions. Typically, the cylinders are deactivated through use of deactivatable valve train components, such as the deactivating valve lifters as disclosed in U.S. patent publication No. U.S. 2004/0244751 A1, whereby the intake and exhaust valves of each deactivated cylinder remain in their closed positions notwithstanding continued rotation of their driving cams. Combustion gases are thus trapped within each deactivated cylinder, whereupon the deactivated cylinders are said to operate as “air springs” while the reduced number of active cylinders operates at a relatively-increased manifold air pressure, with a correlative reduction in engine pumping losses during subsequent engine operation in a partial-displacement engine operating mode. In the meantime, the prior art teaches quickly moving the throttle plate to a post-transition position calculated to provide the requisite mass air flow with which the engine can generate a post-transition torque output roughly matching the pre-transition engine torque output, while fuel and spark is adjusted immediately before and during the transition to further “smooth” torque variations generated during cylinder deactivation.
Upon cylinder deactivation, however, there is a “negative work” component associated with the recompression of the spent combustion gases trapped in the deactivated cylinders, thereby generating additional engine load that must be accommodated in order to prevent a torque disturbance perceptible to the driver. This compression work typically diminishes over several engine cycles as the deactivated cylinders and piston ring packs begin to cool, and as a quantity of such trapped gases blows by the ring packs.